rpk: add the output of 'uptime' to debug bundle

[r-vasquez]

This will be only available on bare-metal deployments, saved in /utils/uptime.txt

$ cat 127.0.0.1-1735943102-bundle/utils/uptime.txt 
 14:25:21 up 1 day,  6:12,  1 user,  load average: 0.76, 0.96, 1.38

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v24.3.x
• v24.2.x
• v24.1.x

Release Notes

Improvements

• rpk: save the output of uptime to bare-metal debug bundles created through rpk.

[Deflaimun]

Should we add something here https://github.com/redpanda-data/redpanda/blob/dev/src/go/rpk/pkg/cli/debug/bundle/bundle.go#L278 about this new file?

[vbotbuildovich]

CI test results

test results on build#60317

test_id	test_kind	job_url	test_status	passed
gtest_raft_rpunit.gtest_raft_rpunit	unit	https://buildkite.com/redpanda/redpanda/builds/60317#01943c8d-1f03-4adb-8ab7-8259f353ab5d	FLAKY	1/2

[sago2k8]

LGTM